On my 77 930 the sunroof is not working (bought the car like that). The electric motor is reacting / 'twitches' but the roof does not move at all.

I recently re-did the seals on the sunroof of a 73 911 so I roughly now my way around. I thought I take down the headliner and dismantle the sunroof to only realize it is a different story when the roof is fully closed...

Hi ..the best start is ..unzip the headliner and remove the motor.Just before this try to see if you can move the cables manually. There is actually tool from Porsche tool kit for this.
So once you get motor off the sunroof will-should drop and you can slide it a little .Then remove the headliner and start investigating....also search is your help here...

Careful pulling that zipper, take your time or you'll tear the headliner. I read a post where the op used chapstick on the zipper. When you get the zipper open remove the cardboard pieces then remove the motor (two screws) then slide the roof back a couple of inches and hook the headliner and pull it down and slide it back. Then take the roof off with a few bolts remove and clean the tracks and cables.